Ummm...Just wanted to say something here that might cause everyone to stop and think. If a mother is identified with a Hep B infection prenatally, it is unknown if she will transmit it to the infant. In these cases it is NOT the Hep B vaccine that is recommended, but the HBIG (immunoglobulin) shot. It is still recommended that the baby gets 3 doses of the Hep B vaccine to prevent FUTURE transmission from the mother out of utero (such as through breastmilk or blood) but the birth dose of Hep B vaccine does not exist to protect the baby from in utero exposure. Only HBIG can do that. It's not known what percentage of babies born to Hep B positive mothers will be born with Hep B, but the (sensible) CDC protocol is to give HBIG to them all. (And Hep B vaccine to cover future exposure, however if they are going to be immediately adopted, and not going to be further exposed to the mother so might not have any greater risk factor to get Hep B in the future, it is not actually needed.)
Think about it if you brought your unvaccinated child to the ER with a tetanus possible wound. They would give them the immunoglobulin (TIG) for the existing wound, then a dose of tetanus containing vaccine (for later wounds, in hope that you would complete the other 2 doses for the series.) It works the same.
I know this because I am a midwife and we had a woman with Hep B when I was a student. It was my job to research what to do and how to obtain HBIG for a birth center birth (next to impossible, she eventually had to transfer for a hospital birth just for the HBIG availability).